It is customary in the art to etch glass (amorphous SiO.sub.2 films) with an aqueous solution of an acid, for example hydrofluoric acid, buffered with, e.g., ammonium fluoride. In the semiconductor industry, it is commonly necessary to etch semiconductor materials like glass formed on a metal substrate, such as aluminum, or to etch glass having metal conducting lines on a silicon wafer or die. The acid and by-products during the etching process attack and corrode the metal.
It has been reported that the addition of a diol alcohol to a buffered acid etching solution inhibits its attack upon the metal. Moreover it has been reported that a suitable amount of diol alcohol, e.g., ethylene glycol, to obtain this result is somewhere on the order of fifty percent of the etching solution For example, it has been stated in U.S. Pat. No. 4,040,897 that a typical formulation using a diol alcohol to inhibit attack upon the metal is comprised as follows: five parts of a buffered oxide etch ("BOE") comprising thirty-four parts by weight of a forty percent aqueous ammonium fluoride solution, five parts of a forty-nine percent aqueous hydrofluoric acid solution, five parts of ethylene glycol and three parts water. In the same patent, it is reported that the diol alcohol containing etchant solution is nearly saturated and that it has a high viscosity which renders it difficult to rinse off and/or to etch small openings in a photoresist. It is reported this formulation tends to corrode the metal intermittently.
U.S. Pat. No. 4,040,897 titled "Etchants for Glass Films on Metal Substrates" discloses a method of etching a glass layer on a metal substrate comprising contacting the glass layer with a dilute aqueous etch solution for a sufficient time to etch through the glass layer. The etch solution disclosed in U.S. Pat. No. 4,040,897 is comprised of a buffered hydrofluoric acid, sodium chloride, and a fluorocarbon surfactant. However, it has been found that using the process disclosed in U.S. Pat. No. 4,040,897 does not completely protect the metal from attack and thus corrosion. Indeed, it has been found that upon exposure to the etchant solution and before the surfactant can coat the metal, the metal is attacked by the acid etchant and by-products and corroded to some extent. In addition, it has been found that the above-discussed processes do not protect the metal substrate from galvanic action which is frequently encountered in wet etching processes with semiconductor materials. More significantly, it has been discovered that the use of salts, like sodium chloride, can under certain circumstances, e.g., during the removal of native oxides on the micro-cathode emitter tips of field emission displays, cause unwanted doping of the silicon substrate by the transference of an ion into the semiconductor. Therefore, there is a need for an etching system wherein metals are protected and the substrate is not unnecessarily doped with, e.g., a sodium ion.